Process for continuously heating hydrocarbon fluids



y 1950 H. D. TROTTER 2,505,871

PROCESS FOR CQNTINUUUSLY HEATING HYDRUCARBON FLUIDS Filed Sept. 29, 1947 PRIMARY IO OIL |N EIILZ PREHEATER DILUENT '53; 5 A :M- QOXYGEN 1 CONXgHNG SECONDARY 21: i123 HEATER 55\ A I7 p. |6\ 5s 5s 23 A LJ\ 52 HOT OIL 62 SEPARATOR CATALYST 4 CATALYST. I CHAMBER 3l CHAMBER SPENT GAS STACK SEPARATION MEANS INVENTOR.

H. D. TROTTER B Y Wm A 7' TORNE KS Patented May 2, 1950 UNITED STATES PATENT OFFICE PROCESS FOR CONTINUOUSLY HEATING HYDROCARBON FLUIDS Harry Darby Trotter, Sweeny, Tex., assignor to Phillips Petroleum Company, a corporation of Delaware 3 Claims.

This invention relates to the removal of carbonaceous deposits from tubes. In one of its more specific aspects it relates toremoval of carbonaceous deposits from tubes in which bydrocarbon oil has been heated in a process for catalytically converting hydrocarbon fluids.

Hydrocarbon fluids, when subjected to elevated temperatures, decompose in part to carbon, coke, or other carbonaceous materials. In heating hydrocarbons to elevated temperatures within a furnace, carbonaceous deposits gradually accumulate on the inner walls of the furnace tubes. The accumulated carbonaceous deposit eventual ly impair the most efficient furnace operation by reducing the cross sectional area and consequently the flow of the hydrocarbon fluids. Furthermore, such carbonaceous deposits reduce the efliciency of the furnace operation by reducing the rate of heat transfer. This difficulty is especially significant in a process wherein heavy oils are heated, a larger quantity of carbon, coke, and other carbonaceous material consequently being deposited on the inner walls of the furnace tube.

The accumulation of a deposit of carbonaceous material, therefore, necessitates its removal and involves taking the tube out of service for cleaning. Various methods proposed for removal of the carbonaceous deposits from the inner walls of heating tubes include oxidation means employing oxygen, or an oxygen-containing gas, and such oxidation means supplemented with meohanical means for dislodging the remaining unburned carbonaceous deposit. In practicing I these methods, it is known that the employment of an oxidation means, can result in temperatures a harmful to the tubes. The employment of a mechanical means, supplementary or otherwise, involves an additional loss of time required for the tube to cool to a temperature at which the equipment can be handled. Such handling involves the removal of headers, return bends, and the like, and because of the frequent handling required, and mechanical abrasive effect on the tubes, effects a shorter life of the mechanical parts.

This invention has for an object a method for removing carbonaceous matter deposited on the inner walls of tubes in a heater employed in a catalytic process for conversion of hydrocarbon oils, whereby the removal of the carbonaceous deposits can be effected without interruption of the continuously operating catalytic conversion process.

A'further object of the invention is the removal of carbonaceous deposits from the inner walls of tubes in a heater employed in a catalytic process for conversion of hydrocarbon oils whereby the removal of said carbonaceous matter can be effected by a novel combustion method wherein severe temperatures, harmful to the tubes, are

. said gas consisting essentially of nitrogen, carbon monoxide, carbon dioxide, and steam, until the duration of the said regeneration is about one-half elapsed, the oxygen content subsequently gradually increasing by virtue of oxygen breaking through the incompletely regenerated catalyst, and continuing to increase until at the termination of said regeneration, said oxygen content is approximately 18 to 20 per cent based on the dry gas effluent.

In accordance with this invention, carbon, coke, or other carbonaceous materials deposited on the inner walls of the tubes within a heating furnace in a process for catalytically cracking hydrocarbon oils, are removed by combustion or a burnout supported by an oxygen-containing gas and diluent comprising the efliuent gas of the catalyst combustion regeneration, supplemented when desired by another oxygen-containing gas and another diluent, the catalyst combustion regeneration gas comprising a mixture of steam, air, and/or another oxygen-containing gas.

As shown in the drawing. the hydrocarbon oil is passed to a primary preheater wherein it is heated to a temperature slightly lower than that at which formation and deposition of carbonaceousmaterial initially occurs, thereby preheat ing the hydrocarbon oil to a maximum allowable temperature and avoiding a deposition of care bonaceous material. The primary preheater effluent is passed intermittently and alternately to one and the other of two secondary heaters wherein said primary preheater efiluent is heated to temperatures required by the subsequent catalytic conversion process, but which are higher than those at which formation and deposition of carbonaceous materials initially occurs, consequently accumulating a carbonaceous deposit on the inner walls of the secondary heater tubes thereby impairing; hydrocarbon fluid flow and eificiency of heat transfer through the tube walls. The primary preheater effiuent is passed to one of two secondary heaters until an accumulation of carbonaceous materials impairs operation. The flow of the primary preheater efiiuent is then switched to the other of the twosecondary heaters thereby effecting a continuation of thesecondary heating step without interruption. Removal of the carbonaceous deposits from the heater tubes previously serving to heat the efliuent of the primary preheaterEis -.eilected simultaneously by combustion supported by effiuent gas of the catalyst regeneration, supplementedlf desired, by another oxygen-containinggas and/or another diluent. The oxygen content of said gas efliuent of the catalyst regeneration is initially nil and subsequent to abouthalf the dur'ation of the catalyst regeneration is in the order of less than two per cent but steadily increases thereby comprising a means for gradually increasing the pxygen content of the combustion supporting gas and-for control of the combustion temperature during removal of said carbonaceous matter.

"Effluent gas of'the catalyst regeneration can be supplemented with another oxygen-containing gas and/or another "diluent, when desired, effecting thereby a further control of.said combustion temperature.

,The intermittent "and alternating operation abovedescribed,provides a methodfor continuing simultaneously the removal of said carbonaceous deposits from tubes in the secondary heating zone and the secondary heating of theprimary 'preheater efliuent.

The'aforedescribed processprovides advantages jyet unmentioned, for example, the process :is

especially advantageous in processingtop crude orheavy gas oils and the like, wherein deposition of carbonaceous matter is greater than that efffected when processing ,a relatively light oil, such as a gas oil having a gravity in the range of '35 or 40 API and an end point of'7 00 E; the intermittent burn-out a'foredescribed prevents an accumulation of'carbon-aceous matter that normally would impair heater operation after an operating period approximating 8 to 12 hours. Another yet unmentioned advantage is aneconomic one, "i. c. a reduced cost of burning out the heater tubes. The ,efiluent catalyst regeneration gas provides a constant source of diluent at the proper temperature and pressure for the burnout, and a source of oxygen for abou'thalf the duration of the catalyst regeneration at the proper temperature and pressure, eliminating to ;a great extent thereby; the cost of compressing vexample of the invention.

Referring now to the drawing, the heavy residual hydrocarbon oil having a gravity in the range of to API and boiling at a temperature in the range ofbilO to 1100"F. is charged to theprimary preheater 9 through line It and heated to a temperature lower than that at which formation and consequent deposition of carbonaceous material occurs, said temperature being in the range of 600 to 900 F. In someinstances [higher temperatures would be permissible de- *pendent upon the characteristics of the specific hydrocarbon oil charged. The preheated oil is passed through eifiuent line I I to either of secondary heaters it or 55. For example, the primary preheater effluent is passed through valve i2 and line with valves i4 and 54 closed, to secondary heater !5 and heated therein to a temperature suitable for subsequent catalytic conversion and rhigher'th'an that at which iormation and consequent deposition of carbonaceous matter initially occurs, said temperature being in the range of 975ll.5il F. and higher, depending on the characteristics of athe specific hydrocarbon oil charged, rand upon the severity of the cracking conditions required. in the subsequent conversion step. The heated oil, primarily in the vapor phase, is passed irom secondary preheater l5 through efiluent line It, valve i8 and line 20, with valve i closed, to hot oil separator 2| and any unvaporized oil or tarry material is discharged through line 22. The vaporous hot charge is then passed through the hot oil separator overhead line -23 and valve El te the-cataiyti-c conversion taking place in either catalyst chamber '3 I or 53. For example, the hot vaporous oil "from valve 2' 3 is passed through line 25 and valve 2? with 'valve's 28,42, and '55 closed, and through line to catalyst chamber 3!. The conversion of hot hydrocarbon vapors takes place at 'a temperature in the range o'f"95l)- 1'000 or higher, and ate pressure'in therangeo'fO-ZOO p. s. i., the actual cracking conditions depending upon the characteristics of the hydrocarbon charge stock, the extent of the conversion desired and, consequen'tlyfth'e severity oithe c'onversion condit ons required. The efiluent "of catalyst chamber 3! leaves through line32, valve 33 with valves and 455 closed, andline '36'to a-separation means "31 forfurther'handling such asfra'c'tionati-on, recycle, etc.

During the conversion, carbon is deposited on the surface of the catalyst, and after a period of time such as "from 2 to 6 hours" continuous operation, an accumulation of said carbon impairs the activity of the catalyst, necessitating the removal thereof. At such a time as th'ecatalyst activity is impaired the i hothydro'carbon vaporous feed to catalyst chamber is switched to an active catalyst in catalyst chamber' iil, and vsubse uent to purging with steam, regeneration of the spent catalyst in chamber 31 is initiated by means of combustion of the carbon deposits accumulated on the surface thereof, said combustion being supported .by a mixture of steam and/0r another relatively inactive diluent with an oxygen-containing gas, said mixture of steam and/or other relatively inactive diluent with .an oxygen-containing gas being referred to .here after as the catalyst combustion regeneration gas. The oxygen content of the efiiuent gas of the catalystcombustion regeneration-is initially substantially nil and a'iteiwaboutone-half of the durati n of the regeneration, said oxygen content is in the order of less than 2%, gradually and progressivelyincreasing until such a time'as said from the inner walls of tubes within heater 55; said catalyst having served previously during a conversion progressing in chamber 43 and said secondary heater 55 having served previously to heat primary preheater eflluent to conversion temperatures approximating 975-1150 F. as aforedescribed. The inactive catalyst is reactivated by the introduction of catalyst combustion regeneration gas under conditions efiecting the removal of said carbon, to said catalyst chamber 43 through lines 38 and 4| and valve 40 with valves 28, 39 and 42 closed, and removal of carbonaceous deposits from within heater 55 is effected by a burn-out, or combustion. supported by efiluent gas of the catalyst regeneration taking place in catalyst chamber 43, and supplemented if desired with another oxygen-containing gas and/or another diluent, said eflluent gas leaving catalyst chamber 43 through line 44 and valve 46 with valves 45 and 34 closed and through line 41 splitting in part to line 48 through valve 49 to spent gas stack 55 and continuing in remaining part to secondary heater 55 through line 41 and valves 52 and 53 with valves 14 and 54 closed. The efiiuent of the burn-out progressing in secondary heater 55 leaves through lines 56 and 5'! and valve Ell with valves 58 and 6! closed, to spent gas stack 50.

In the practice of th s invention, temperature control of combustion during the burn-out of the secondary heaters is eff cted to a major extent by pa sing effluent gas of the catalyst regeneration throu h said heaters and to a minor extent by supplementing said efiiuent gas with another oxygen-containing gas and/or another diluent gas such as steam. Said efliuent gas has an oxygen content of substantially nil during at least the first hour of the burn-out and compri es during a period thereof. a m xture of nitrogen, steam and carbon dioxide, thereby serving as a diluent of any supplementary stream of oxygen-containing gas that might be employed. The oxygen content of said efiluent gas after a time, as aforedescribed, becomes significant and gradually increases to a maximum. and during that period, said efiiuent gas functions as a source of both oxygen and diluent thereby gradually decreas ng to substantially nil the quantity of supplementary oxygencontaining gas being utilized as aforedescribed. It is possible that during the latter stages of said catalyst regeneration. at the time the'oxygen content of said effluent gas approaches the aforesaid maximum of 18-20%, supplementary diluent would be ut lized to maintain the proper ratio of oxygen and diluent required to avoid local overheating within said secondary heater and consequent harmful effects on the heater tubes. The employment of said supplementary diluent and oxygen-containing gas streams varies as a function of the coking characteristics, or tendency of the specific hydrocarbon oil charged to deposit carbonaceous materials. Th s is obvious as the quantity of deposited carbonaceous material withl in the heater determines the quantity of oxygen required for removal of samejsaid quantity of oxygen in turn determines the quantity of diluent required to maintain a minimum ratio of diluent the effluent gas of the catalyst regeneration, within the desired duration, usually that of the catalyst regeneration period. no supplementary oxygen 6 gas would be required. Probably no supplementary diluent would be required.

When a heavy residual oil, 1. e. one having gravity in the range of 20-30 API and boiling in the range of 600-1000 F., has been charged to the secondary heaters it is advisable to initially employ supplementary oxygen-containing gas and subsequently employ supplementary diluent gas in order to effect complete removal of the consequent deposition of carbonaceous matter within the duration of the catalyst regeneration period, at the end of which, the removal of all carbonaceous deposits within the secondary heater would be complete.

When charging a hydrocarbon oil havinga gravity in the range of 35-40 API and boiling in the range of 400-750 F., it is unnecessary to employ supplementary oxygen-containing gas and/or supplementary diluent to effect complete removal of consequent depositions of carbonaceous matter. In such a case the effiuent gas of said catalyst regeneration, initially comprising nitrogen, carbon diox de, and steam, as aforedescribed, and subsequently comprising in addition to aforesaid, a gradually increasing content of oxygen as aforedescribed provides thereby the necessary quantity of oxygen for removal of said carbonaceous deposit and a sufficient quantity of diluent to ma ntain the proper aforesaid ratio of diluent and oxygen. such that the carbonaceous deposits will be completely removed from the secondary heater within the duration of the catalyst regeneration.

In any instance, said eflluent gas of catalyst regeneration can be used independently of supplementary oxygen-containing gas, by extending said burn-out to twice or even several times the duration of the catalyst regeneration period, supplyin thereby the required quant ty of oxygen and diluent for the complete burn-out of said secondary heaters.

Oxygen-containing gas, supplemental to the effluent gas of the catalyst regeneration supporting the burn-out or combustion of the carbonaceous deposits within the secondary heaters, as aforedescribed, can be introduced through valve 68 to line 41. Supplementary diluent gas can be introduced through valve 66 to line 41.

When the deposi ion of carbonaceous material upon the inner walls of the tubes compri ing the coil in heater I5 becomes such as to necessitate reconditioning the tubes, i. e. removing the accumu ated deposits therefrom, the removal of carbonaceous deposits in heater 55 previously serving to heat primary preheater effluent to the conversion temperatures, aforedescribed, has been completed.

Reconditioning of the tubes in s condary heater I5 is initiated subse uent to diversion therefrom of preheater 9 effiuent and introduction thereto of aforesaid effiuent catalyst regeneration gas, supplemented with another oxygencontaining gas and/or another diluent if desired, by means of passing efiluent regeneration gas of catalyst chamber 43 to the spent gas stack through lines 44, 4! and 48 and valves 46 and 49 with valves 45, 34 and 5| closed, passing effluent of primary preheater 9 to secondary heater 55 through line I I and valve 54 with valves 53 and !2 closed, passing effluent of secondary heater 55 to catalyst chamber 3! through lines 56, E4, 62, 25 and Si) and valves 5|. and. 21 with valves 60. 24, 28 and 42 closed, and passing *efliuent of catalyst chamber 3! to a separation-means as aforedescribed. Heater I5 is then purged with ing part through lines d1,

. J tently,

vsteam introduced to -line 4 through valve ,66 and passed through valve M and line it, with valves E53 .and I2 closed, the effluent therefrom being passed to the spent .gaszstack through line it, waives :H and 58 and line :5? with valves 13 and r J51] closed. Subsequent to a brief purge period thereof, the ilow :of catalyst combustion regenaeration gas is diverted from .catalyst chamber 43 directly to -.heater Hi by closing .valve -50 with 'rvalve 39 openrand passing said regeneration gas through 'linesd'i and it and valves 52 and 14 iwithvalvesiil, tifiand it closed, and passing the :effluent gas-therefrom to the spent gas stack through lines it and 5'! as aforedescribed. The

flow f hydrocarbon-effluent of heater 55 is then sdiverted from catalyst chamber 5:! to catalyst chamber A3 by closing valve with valve d2 'open, and passing eiiluent through line 3!. ilhe:efiiuent of catalyst chamber can thenpassed .3? tl'n'cmgh'line" and gas is diverted frornheater iii directly to'catalyst chamber 3'! by openingvalve 2t withvalvesfil, 6'1, 39 and closed and passin; said regenera- "tion gas throughline Effiuent catalyst combustion regeneration gas of cheznber 3'! is then passed to heater through line 32 and valve '34 through line t? with valves and 135 "closed, splitting in part through line es and valve 49 "to spent gasstack. cc and ccntinuingin remain- !3 and valves 5!,52 .and M with valves 38, 53 and I2 closed. Supplementary oxygen-containing gas can be introduced to line Ail through valve "63 and supple- "mentary diluent, such as steam, can be introducedthrough valve 55 to line 45?. Efi'luent'com- 'bustion ,gas of heater id is passed to spent gas stackfic'thrcugh line 5! by means aforedescribed.

"Similarly, preheater 9 efiluent is subsequently switched back to secondary heater l5, heated therein and converted in catalyst chamber 3i,

7 while the fouled catalyst in chamber 3 is re generated and carbonaceous deposits removed from secondary heater 55.

The aioredescribed switch is effected intermitand comprises thereby, a continuous meansof heating the hydrocarbon charge to the cataiytic conversion process, without an inter-- rupti'on to the catalytic conversion that would result normally from an accumulation of ca .hcnaceons. deposits within the. heating tubes, icr vexample, an impairment of fluid flow by virtue ..-of areduced crease-notional area of said tube ,derstood that this invention is limited thereby as it is obvious that-catalyst regeneration and sec- -oniiary heater burn-cut cycles aforementioned,

canoperate independently of each other.

It is not intended that the representation of ":the equipment as shown in the accompanying :drawing be limited to any specific type used in .a: specific process, vfor the method of the invention.isapplicable to any of the ,many types of heating. arrangements used .catalytic conver- SiOHpI'OCBSS GSOf. hydrocarbon fluids. I

My invention ,is illustrated and described in terms of a ,fixed bed catalyst. However, it is'to be understood that my invention is not to be limited thereby, for it can be practiced in any catalytic hydrocarbon conversion process wherein the formation and deposition of carbonaceous materials on the surface of the catalyst is concornitant with conversion of said hydrocarbon, inactivating said catalyst thereby, and wherein .theactivity of the catalyst is regenerated 'bythe removal of said carbonaceous material by means of a combustion or burn-out, as for xample, a. hydrocarbon conversion process employing :a fluidized catalyst. H

Havingifull described my invention, I claim:

1. An improved process for continuouslyfheat- 'ing and converting a hydrocarbon oil having' a gravity the range of 35 to 40 A. P. I. andbolling in the range of 400 .to 750 F., in the presence of a granular fixed bed catalyst requiring regeneration by a combustion with an oxygen-containinggas. which comprises continuously preheating a stream of said oil to a temperature below that at which carbonaceous maerials deposit from said stream; passing said preheated hydrocarbon stream'to one of a plurality of secondary heating zones and further heating same therein "to a temnerature in the range of that'requiredby-subsequent conversion, with concomitant deposi ion of carbonaceous materials; passing the thus heated hydrocarbon stream to client a plurality of catalytic reaction zones containing an active granular fixed-bed catalyst wherein said hydroa carbon is converted, said catalyst'becoming progressivel inactive by formation and deposition of carbonaceous materials on the surface -thereof; passing efiluent of said catalyst zon to a separation means. subsequentlyheating said. preheated stream of oil in another of said plurality-pisseondary heating zones to said convers'icntempera- 'ture, passing the hydrocarbon stream to another of said plura it of said catalytic reaction "zones and converting same therein and passing resulting eflluents to said separation means, simultaneously passing regeneraion gas containing from 5 to 20 per cent free oxygento the inactive catalyst 'inthe first said catalytic reaction zone in a direction concurrent with theaforesaidflow of oil being cracked, and under conditions effecting removal of carbonaceous material -deposied thereon and producing effluent regeneration gas from first-said catalytic reaction zone initialiy containing no free oxygen'and subsequent- 1y containing free oxygen unz'ea cted duringsaid regeneration in progres ively increasing concentrationsuntil when carbonaceous matter is substantiall completely removed from the surface of said inactive catalyst the oxygen content of said efiluent regeneration gas is substantiallythe same as thatoi said regeneration gasbeing'passed oxygen therewith and without ,ad-mixinga diluent.

gas therewith in a direction-concurrent with-the aforesaid flow of :oil being heated under: c0nd1- tions effecting reaction of oxygen .with carbonaceous materials deposited therein; and vremoving from the system residual carbonaceous I secondary zone ascom- .bustionproduct.

g .2. An improved process .ior continuously heat- 9 ing and converting a hydrocarbon oil having a gravity in the range of 35 to 40 A. P. I. and boiling in the range of 400 to 750 F., in the presence of a granular fixed bed catalyst requiring regeneration by a combustion with an oxygen-containing gas, comprising continuously preheating a stream of said oil in a primary preheater zone to a temperature below that at which carbonaceous materials deposit from said stream, passing preheated hydrocarbon stream from said primary preheater zone to a first secondary heater zone and therein further heating same to a temperature in the range of that required by the subsequent conversion, with concomitant deposition of carbonaceous materials; passing the thus heated hydrocarbon stream to a first catalytic cracking zone containing a fixed bed of active granular bauxite catalyst wherein said hydrocarbon is converted, said catalyst becoming progressivel inactivated by formation and deposition of carbonaceous materials on the surface thereof; passing efiluent of said first catalyst zone to a separation means; subsequent to said catalyst becoming inactivated, terminating the flow of heated hydrocarbon from said primary preheater to said first secondary heater zone and passing said heated hydrocarbon to a second secondary heater zone and therein heating same to the temperature required by said conversion; passing the hydrocarbon stream thus heated in the secondary hydrocarbon zone to a second catalytic reaction zone containing an active granular fixed bed catalyst of the kind contained in said first catalytic zone and therein converting said hydrocarbon and passing resulting efiluents to said separation means; concurrent with converting hydrocarbons in said second catalytic zone admixing air with steam to produce a resulting steam-air admixture containing at least five volume per cent oxygen and passing the resulting steam-air admixture through the inactivated bauxite catalyst bed in said first catalytic zone in a direction concurrent with the aforesaid flow of oil being cracked and therein regenerating inactivated catalyst by burning carbonaceous deposits free from the catalyst surface; effluent regeneration gas of said first catalytic zone initially containing no free oxygen and subsequently containing oxygen unreacted during said regeneration in progressively increasing concentrations until when carbonaceous matter is substantially completely removed from the surface of said inactive catalyst the oxygen content of said efiluent regeneration gas is substantially the same as that of said steam-air admixture being passed into said first catal tic reaction zone; concurrent with heating hydrocarbons in said second secondary heater zone passing said efiiuent regeneration gas from said first catalytic zone, initially containing no free oxygen and subsequently containing oxygen, through said first secondary heater zone in a direction concurrent with the aforesaid flow of oil being heated without supplementing same with oxygen and without supplementing same with inert diluent, and burning free said carbonaceous deposits therein; and discharging all the gaseous eflluent from said concurrent flow of regeneration gas through said secondary heater zone.

3. The process of claim 2 wherein said hydrocarbon oil is heated to a temperature below about 900 F. in said primary preheater zone and to a temperature in the range of 900-1300 F. in each said secondary zones.

HARRY DARBY TROTTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,470,359 Greenstreet Oct. 9, 1923 1,769,789 Leamon July 1, 1930 2,199,837 Murphree May 7, 1940 2,199,838 Tyson et al May 7, 1940 2,210,257 Pyzel et al. Aug. 6, 1940 2,338,581 Guyer Jan. 4. 1944 

1. AN IMPROVED PROCESS FOR CONTINUOUSLY HEATING AND CONVERTING A HYDROCARBON OIL HAVING A GRAVITY IN THE RANGE OF 35 TO 40* A. P. I. AND BOILING IN THE RANGE OF 400* TO 750*F., IN THE PRESENCE OF A GRANULAR FIXED BED CATALYST REQUIRING REGENERATION BY A COMBUSTION WITH AN OXYGEN-CONTAINING GAS, WHICH COMPRISES CONTINOUSLY PREHEATING A STREAM OF SAID OIL TO A TEMPERATURE BELOW THAT AT WHICH CARBONACEOUS MATERIALS DEPOSIT FROM SAID STREAM; PASSING SAID PREHEATED HYDROCARBON STREAM TO ONE OF A PLURALITY OF SECONDARY HEATING ZONES AND FURTHER HEATING SAME THEREIN TO A TEMPERATURE IN THE RANGE OF THAT REQUIRED BY SUBSEQUENT CONVERSION, WITH CONCOMITANT DEPOSITION OF CARBONACEOUS MATERIALS; PASSING THE THUS HEATED HYDROCARBON STREAM TO ONE OF A PLURALITY OF CATALYTIC REACTION ZONES CONTAINING AN ACTIVE GRANULAR FIXED-BED CATALYST WHEREIN SAID HYDROCARBON IS CONVERTED, SAID CATALYST BECOMING PROGRESSIVELY INACTIVE BY FORMATION AND DEPOSITION OF CARBONACEOUS MATERIALS ON THE SURFACE THEREOF; PASSING EFFLUENT OF SAID CATALYST ZONE TO A SEPARATION MEANS, SUBSEQUENTLY HEATING SAID PREHEATED STREAM OF OIL IN ANOTHER OF SAID PLURALITY OF SECONDARY HEATING ZONES TO SAID CONVERSION TEMPERATURE, PASSING THE HYDROCARBON STREAM TO ANOTHER OF SAID PLURALITY OF SAID CATALYTIC REACTION ZONES AND CONVERTING SAME THEREIN AND PASSING RESULTING EFFLUENTS TO SAID SEPARATION MEANS, SIMULTANEOUSLY PASSING REGENERATION GAS CONTAINING FROM 5 TO 20 PER CENT FREE OXYGEN TO THE INACTIVE CATALYST IN THE FIRST SAID CATALYTIC REACTION ZONE IN A DIRECTION CONCURRENT WITH THE AFORESAID FLOW OF OIL BEING CRACKED, AND UNDER CONDITIONS EFFECTING REMOVAL OF CARBONACEOUS MATERIAL DEPOSITED THEREON AND PRODUCING EFFLUENT REGENERATION GAS FROM FIRST SAID CATALYTIC REACTION ZONE INITIALLY CONTAINING NO FREE OXYGEN AND SUBSEQUENTLY CONTAINING FREE OXYGEN UNREACTED DURING SAID 